This invention relates to the treatment of gas streams. In particular, it relates to the treatment of a gas stream comprising hydrogen sulphide.
Gas streams comprising hydrogen sulphide are typically produced as waste products or by-products from many industrial processes. For example, acid gas streams comprising carbon dioxide and hydrogen sulphide are typically produced during oil refinery operations in which sulphur is removed from crude oil. It is necessary to treat such hydrogen sulphide-containing streams before discharging them to the atmosphere so as to reduce or remove altogether their content of sulphur-containing gases. One well known, widely practised process for treating a gas stream comprising hydrogen sulphide is the Claus process. This process is based on the reaction between hydrogen sulphide and sulphur dioxide to form sulphur vapour and water vapour in accordance with the equation. EQU SO.sub.2 +2H.sub.2 S=2H.sub.2 O+3S
Sulphur exists in the vapour phase in a number of different molecular species such as S.sub.2, S.sub.6 and S.sub.8 according to the temperature.
The first stage of the Claus process is to burn approximately a third of the hydrogen sulphide in the incoming gas stream to form sulphur dioxide and water vapour in accordance with the equation: EQU 2H.sub.2 S+3O.sub.2 =2H.sub.2 O+2SO.sub.2
This combustion reaction takes place in a suitable furnace and normally air is used as the source of oxygen for the purposes of combustion. The furnace is designed such that reaction between the sulphur dioxide and hydrogen sulphide can start in the combustion zone and then continue downstream of the combustion zone. It is however a feature of the Claus reaction that at the temperature that is created by the combustion of hydrogen sulphide, it is not possible to convert more than about 75% of the remaining hydrogen sulphide to sulphur by reaction with sulphur dioxide, and typically between 50 to 70% of the hydrogen sulphide is so converted. It is however possible to achieve a higher percentage conversion in the presence of a catalyst at a reaction temperature in the order of 200.degree. to 350.degree. C. by reacting the remaining hydrogen sulphide and sulphur dioxide. (At such "catalytic" temperatures, the lower the temperature the higher is the percentage conversion that is achieved). Accordingly, after the gases pass out of the so-called thermal region of the furnace they are cooled to a temperature at which the sulphur that is formed in the furnace condenses. The sulphur is thus recovered. The gases are then reheated to a temperature suitable for the performance of a catalysed reaction between hydrogen sulphide and sulphur dioxide, such temperature typically being in the order of 200.degree. C. A catalytic reaction is then carried out and typically about 60% of the remaining hydrogen sulphide is converted to sulphur. Nonetheless, it is still not possible to achieve 100% conversion as in practice conversions of more than 99.5% can be achieved only at a temperature at which the sulphur vapour condenses and thereby substantially reduces the effectivess of the catalyst. It is therefore typical to perform the catalytic oxidation of hydrogen sulphide with sulphur dioxide in more than one stage with first condensation of sulphur vapour and then re-heating of the hydrogen sulphide bearing gas stream being carried out between each stage.
Various means may be employed to effect reheating of the gases prior to each catalytic stage. For example, a small part of the feed gas mixture can be diverted from upstream of the furnace and burnt in in-line burners completely to sulphur dioxide, there being typically one such burner upstream of each catalytic reactor. The hot, sulphur dioxide-containing gases are then mixed with the main gas stream upstream of each respective catalytic reactor so as to effect reheating. Alternatively, a part of the main gas stream can be taken from, say, a waste heat boiler used to cool the main gas stream leaving the furnace and used in the same manner as the gas from the in-line burners. Another alternative is to employ indirect heat exchange with, for example steam to effect reheating. Typically, after two or three such stages, sulphur formed in the most downstream stage is condensed out of the gas stream which is then passed to a tail gas clean-up process of a known kind for handling relatively dilute hydrogen sulphide streams (for example the Scot, Beavon or Stretford process) or which is then incinerated.
Many variations on this basic Claus process are possible. Some of these alterations are summarised in the paper "Sulfur Costs vary with Process Selection" by H. Fischer, Hydrocarbon Processing, March 1979, pp125 to 129.
Recently, there has been a trend towards using crude oils of relatively high sulphur contents and also a trend towards stricter environmental standards so far as the discharge to the atmosphere of sulphur-containing gases is concerned, thus requiring an increased number of hydrogen sulphide bearing streams to be treated and hence more treatment capacity for hydrogen sulphide containing gases. For example, where possible, it is desirable to increase the rate at which an exising Claus plant is able to produce sulphur. In practice, the ability of such plants to handle an increased throughput of hydrogen sulphide containing gas is limited. It has been realised that in order to supply the necessary oxygen for combustion, approximately 14 volumes of air are required for each six volumes of hydrogen sulphide in the gas mixture. It has been proposed in for example a paper entitled "Oxygen Use in Claus Sulphur Plants" by M. R. Gray and W. Y. Svrcek, 1981 Gas Conditioning Conference, Oklahoma, 1981 and in a paper entitled "Modifications Jump Sulphur Recovery Plant Capacity", Oil and Gas Journal, Aug. 20, 1984, pp108 to 112, that the capacity of existing Claus processes can be increased by substituting some commercially pure oxygen for air and thereby reducing the proportion of nitrogen in the gas mixture that flows through the process. In practice, however, in many plants, the amount of uprating that can be achieved by this method is limited as there is a tendency for the reduced volume of nitrogen to lead to higher exit temperatures from the furnace that cannot be withstood by the waste heat boiler or heat exchanger associated with the furnace or by the refractory lining of the furnace. Indeed, the more concentrated (in hydrogen sulphide) the gas stream, the less is the possibility for achieving any significant uprating, such possibility often becoming particularly limited for feed gas streams including 80% by volume or more of hydrogen sulphide. Another proposal for using pure oxygen in the Claus process is set out in U.S. Pat. No. 3,681,024 and its corresponding Canadian patent specification 854094. These patent specifications disclose burning one third of a hydrogen sulphide stream with oxygen of about 95% purity. Plant effluent from a one or two catalytic reactor unit is sent to a water scrubber to reduce the water content of the effluent, and a sufficient amount of the scrubber off-gas is recycled to dilute the oxygen feed so that the furnace temperature is essentially equivalent to that obtained in operation with air.
This process is stated to have the advantage of enabling plant size to be reduced. However, existing plants constructed with the intention of using air to support the combustion of the hydrogen sulphide are not readily convertible to perform the process described in U.S. Pat. No. 3,681,024 and this process has not found commercial layout. Moreover, the practice of recycling to the thermal reaction zone a gas mixture that has passed therethrough places a limitation on the amount by which the size of the furnace defining the thermal reaction zone can be reduced, particularly if the incoming hydrogen sulphide stream contains more than, say, 50% by volume of hydrogen sulphide. U.S. Pat. Nos. 3,331,733 and 4,552,747 are other examples of proposals in which gas is recirculated in order to moderate the temperature in the thermal reactor.